The present invention relates to an operable membranes device and a digital speaker comprising at least one such device.
A digital speaker comprises operable membranes moving air in order to emit sounds.
The development of nomadic devices always offering more functionalities, ever higher performing components, and increasingly integrated are being developed. MEMS components, due to their collective manufacturing method, and the potential for co-integration of different components on the same chip are opening up new prospects for industrialists.
Speakers are components present in a large number of mobile telephone, flat screen, etc. applications, and their miniaturization and integration are becoming key.
MEMS technologies are particularly well suited to producing digital speakers, in which the sound is reconstructed from elementary contributions from a large number of elementary membranes, and more generally ultrafine acoustic transducers or speaklets. Each transducer can be actuated independently, and the sound to be emitted is reconstructed based on the principle of additivity of the elementary sounds of transducers in the air.
One example digital speaker is described in the document Dejaeger et al. “Development and characterization of a piezoelectrically actuated MEMS digital loudspeaker”, Proc. Eurosensors XXVI, Sep. 9-12, 2012. In this document, the speaklets use piezoelectric actuation, which makes it possible to give a movement to the speaklets in both directions, and therefore to precisely control the acoustic pulse that they generate.
Nevertheless, it has been observed that the elementary MEMS membranes generate fewer low frequencies when the resonance frequency of the membrane is high. Indeed, if we trace the variation of the surface pressure as a function of the frequency, we see that, before the resonance peak, the pressure increases. This increase, of approximately 12 dB per octave, is related to the mechanical behavior of the membrane, which is dominated by the stiffness. This mechanical behavior approaches a mass-spring system with one degree of freedom. Beyond the resonance peak, the pressure remains constant, since the behavior of the membrane is dominated by the mass.
Furthermore, the resonance frequency of the membrane is higher when the membrane is smaller with a constant thickness. In order to generate low frequencies, it is therefore preferable to implement large membranes.
However, when the speaklet matrix is too large, for example when its diameter is larger than 2500 μm, distortions may appear due to operating differences, i.e., the difference in the journey of the acoustic wave between two transmitters.
Furthermore, the membranes are also subject to residual constraints. In the case of speaklets, the generated resonance frequency is higher when there are residual constraints. Consequently, the presence of these residual constraints hinders the generation of low frequencies.
Furthermore, the existence of these residual constraints, both in digital speakers and other devices implementing operable membranes, makes the behavior of the membranes unpredictable and their control imprecise. Indeed, due to the presence of these residual constraints, the membrane system does not have the anticipated behavior for a planar membrane system.
In light of the preceding, in the case of a device having a large number of membranes or membranes of different sizes, these membranes will therefore have constraint states that may in particular vary as a function of their geographical position on the substrate, and it is not possible to consider using a manufacturing method making it possible to offset these drawbacks for all of the membranes.